Disneyland's get in for free on your birthday promotion ends December 31, 2009.
If you're reading this before then, just register ahead of time and they'll send an e-mail confirmation. Take it and an ID to the ticket booths to claim your free entry. It's good for one park only, but you can upgrade by paying the difference in cost.
To my happy surprise, I recently discovered that if you already have a multi-day Disneyland ticket or a season pass, you can get a gift certificate for the value of the one-day, one-park ticket.